HEAVY RESIDUE PRODUCTION IN HEAVY ION INDUCED REACTIONS ON 124 Sn BETWEEN 10 AND 84 MeV/NUCLEON

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HEAVY RESIDUE PRODUCTION IN HEAVY ION

INDUCED REACTIONS ON 124 Sn BETWEEN 10

AND 84 MeV/NUCLEON

A. Lleres, J. Crançon, J. Blachot, A. Gizon, H. Nifenecker

To cite this version:

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HEAVY RESIDUE PRODUCTION IN HEAVY ION INDUCED REACTIONS ON 12kSn BETWEEN 1 0 A N D 84 MeV/NUCLEON

A. L L E R E S , J. CRANgON*, J. BLACHOT*, A . GIZON and H. NIFENECKER*

Instltut des Sciences Nuclgaires, IN2P3, 53, a v e n u e d e s Martyrs, F-38041 Grenoble, France

'CENG/DRF, Service de Physique, Ph.N, BP 85 X, F-38041 Grenoble Cedex, France

Résumé - Les spectres en vitesse des résidus lourds émis dans les réactions C, ZUNe, 40Ar + 124Sn ont été mesurés entre 0° et 10°, à plusieurs énergies du do-maine 10-84 MeV/nucléon. Les données sont en accord avec l'hypothèse d'un pro-cessus de fusion incomplète asymétrique.

12 20 40 124

Abstract - Heavy residue v e l o c i t y s p e c t r a of C, Ne, Ar + Sn r e a c t i o n s induced i n the 10-84 MeV/nucleon energy range have been measured between 0° and 10°. Asymmetric incomplete fusion process c a l c u l a t i o n s a r e i n agreement

with the data.

A limitation of the linear momentum transfer in central heavy ion collisions has been observed in a lot of various experiments performed at intermediate energy /I,3/. In other words, the complete fusion process becomes less and less probable as the incident energy increases. To follow progressively the evolution of the more cen-tral processes, we have focused our effort on a series of systematic velocity mea-surements of 1 2 4Sn target-residues recoiling between 0° and 10°. The aim is

topreci-se, as far as possible, the respective parts of projectile and target participating to the formation of the fused systems which precede the emission of heavy residues. In this paper, we present in details a set of results on the 20Ne + 124sn

sys-tem and we apply simple incomplete fusion calculations, in the frame of reasonable assumptions, to reproduce the data at the best. Then, the treatment is extended to mass-velocity spectra observed for other systems as <2c + 124§n o r 4 0A r + 1Z4sn.

The experiments were performed with various heavy ions, at different energies. The beams were delivered by several accelerators (ALICE : 10 MeV/nucleon, SARA : 20-30 MeV/nucleon, GANIL : 27-60 MeV/nucleon, SC at CERN : 49 and 84 MeV/nucleon).

The technique was based upon off-line gamma-activity measurements following target irradiations associated with on-line collection of reaction products /4/. Heavy residues emitted around the beam axis were collected by a stack of thin aluminium foils (1 or 2 vm thick) set behind a thin (400 ug x cm- 2) target, perpendicularly to the

beam axis. A collimator placed between the target and the catchers defined a solid angle of 95 msr (9 = 0° - 10°). The identification of the reaction products with 70 4 A 4 130 masses was precisely established from the analysis of Y ~r a v activity

mea-surements of all catchers. The velocity distributions were extracted, by use of range tables.

The mass-velocity spectra obtained for residues emitted between 0° and 10° in the 2 0Ne + 1 2 4Sn reaction at 20, 30, 40 and 49 MeV/nucleon are shown in figure 1,

On these diagrams, one observes clearly that, at each bombarding energy, the most pro-bable velocity of the residues, Vp, although relatively high, is lower than the; cen-ter of mass velocity Vcm- This indicates that, for all incident energies, the final products correspond to evaporation residues coming from incomplete fusion mechanisms. Moreover, with increasing incident energy, the difference between Vp and Van increa-ses, indicating that the involved momentum transfer is more and more incomplete.

As the residues have been measured between 0° and 10°, they have been

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ted with p a r a l l e l momentum t r a n s f e r respect t o the beam axis (pA 9 0) and the velo- c i t y of the formed fusion-like nucleus i s not s i g n i f i c a n t l y perturbated by the f o l l o - wing evaporation (Vre5idue z Vfusion-like nucleus)

.

In f i r s t a~proximation, the expe-

Fig. 1

-

Mass-velocity spectra of residues emitted between 0" and 10" f o r 2 0 ~ e + 1 2 4 ~ n a t 20,30,40 and 49 MeV/nucleon. Vcm is the center of mass velocity ; Vm is the most probable velocity measured f o r the laver masses and Vp i s the most probable velocity of the distribution integrated over a l l masses. Vp(A> ( - - - ) represents the evolution of the measured most probable energy as a function of mass ; see the t e x t f o r the curves labelled a,b (-) and c , d ( - - - ) .

rimental r e s u l t s can be compared with V and A values calculated i n simple incomplete fusion models where only a few parameters are used. To estimate the velocity V and the mass A of the residues, several assumptions can be done t o r e a l i z e the formation of the fusion-like nucleus.

In the " g g p e t r i c l y p o t h e s i ~ _ ~ ~ , a p a r t of the p r o j e c t i l e fuses with the tota- l i t y of t h e target. The spectator nucleons of the p r o j e c t i l e a r e emitted (asnucleons, c l u s t e r s o r unique quasi-projectile fragment) i n the beam direction, with the projec- t i l e velocity V (case labelled a) o r with a p a r t of the incident velocity (for exam- p l e 80 %, case Pabelled b)

.

In t h e l l s ~ e t r i c - & ~ ~ t & e _ s _ ~ s _ t ' , a p a r t of the p r o j e c t i l e fuses with a p a r t of the t a r g e t with the assumption t h a t the same number of nucleons a r e escaping from the target a s from the p r o j e c t i l e . In t h i s hypothesis, two reasonable s i t u a t i o n s have been t r e a t e d f o r the spectator nucleons : i n t h e f i r s t case they a r e emitted along the beam a x i s with t h e i r i n i t i a l v e l o c i t i e s , namely Vp, for the p r o j e c t i l e nucleon

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the velocity V and the mass A of the residues from the momentum and energy conserva- t i o n relations /3/.

Calculations of velocity and m a s s values have been done f o r theresiduesprodu- ced i n the 2%le + reaction a t 30 MeV/nucleon i n the four above mentioned cases and with a number of involved p r o j e c t i l e nucleons varying from 0 t o 20. The r e s u l t s a r e drawn i n figure 1 f o r the 30 MeV/nucleon mass-velocity spectrum. I t appears c l e a ~ r l y t h a t the calculated curve labelled "a" f i t s q u i t e well the experimental A,Vdistr& bution while the three other estimations disagree significantly.

To lighten the 20, 40 and 49 MeV/nucleon mass-velocity diagrams, the s o l i d li- nes drawn correspond t o values calculated i n the frame of the "asymmetric hypothesis" and case "a" ( f i g . 1 ) . One sees t h a t the agreement between the experimental A,V distributions and the calculated ones i s u i t e good.

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In conclusion, f o r the 2 0 ~ e + 1 4 ~ n system, from 20 t o 49 MeV/nucleon, an incomplete fusion process where a p a r t of the p r o j e c t i l e fuses with the t a r g e t while the spectator nucleons a r e emitted along the beam'axis with the incident v e l o c i t y , i s i n agreement with the experimental mass-velocity spectra of the residues observed between 0" and 10". Especially, the evolution of the d i s t r i b u t i o n s , with increasing incident energy, i s remarkably well reproduced. Nevertheless, as the measurements have been made i n the 0"-10~ range, reactions with very small perpendicular momentum transfers have been predominantly selected and s o , we cannot say t h a t a l l incomplete fusion collisions lead t o the emission of f a s t particules having the p r o j e c t i l e velo- c i t y . One can only conclude t h a t t h e "asymmetric hypothesis" i s probably the b e s t a s - sumption f o r the whole of the incomplete fusion collisions.

Fig. 2 Fig. 3

Same as f i g . 1 f o r other systems.

To check the applicability of t h i s hypothesis, comparisons have been made f o r A,V experimental residue distributions measured with various projectiles. T i c a l examples a r e reported f o r 1% + 124sn a t 84 &V/nucleon (fig. 2) and 4 0 ~ r +y24Sn a t 24 MeV/nucleon (fig. 3 ) . In both cases, the "asymmetric hypothesis" remains a good approach t o f i t the experimental mass-velocity spectra a t small angles (0'-10"). The comparison i s specially i n t e r e s t i n g i n the 4 0 ~ r + 1 2 4 ~ n case. Between 24 and 35 MeV/ nucleon, the velocity spectra exhibit two d i f f e r e n t components. From the comparison between A, V experimental r e s u l t s and "asymetric hypothesis" calculations, the si-

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REFERENCES

/1/ Tsang,M.B. e t a l . Phys. Lett. 134B (1984) 169. /2/ Lefort, M., IPNO/DRE 85-15 and r e f . i n .